The present invention relates to an improvement in an intake system for use in an internal combustion engine such as four cycle engine or two cycle engine in the principal respect for small vehicle such as automotive bicycle or the like.
In general, the opening degree of the valve required for the intake stroke is set so as to be rather large, thereby bringing about the intake inertia effect, enhancing the charging efficiency and increasing the output in the internal combustion engine.
On the other hand, if the opening degree of the valve required for the intake stroke is relatively large in the low rotation range where there is little intake inertia effect, there may occur the backflow of the air-fuel mixture, thereby lowering the charging efficiency.
In order to prevent this backflow of the air-fuel mixture, there has been known an internal combustion engine with a one-way valve. The one-way valve such as reed valve is provided in an intake passage bridging between a carburetor and a combustion chamber. The present applicant has already proposed an engine of this type (Japanese Patent Publication No. 50884/1972) which is provided with a throttle valve in an intake passage which is in communication with the cylinder of the engine by way of an intake valve as well as a check valve which is positioned at the downstream side thereof so as to permit the inflow of the air-fuel mixture into the combustion chamber but prevent the backflow of the air-fuel mixture, thereby enhancing the output characteristic of the engine especially at the low speed range. However, the intake system of this kind has a drawback in that it takes several time during the intake stroke for the negative pressure in the combustion chamber to be transmitted to the throttle valve in the carburetor by way of the intake passage, thereby disabling the instantaneous feeding of the air-fuel mixture.
In addition, inasmuch as the natural vibration frequency of the reed valve is set higher than the frequency to open and close the valve per hour along with the rotation of the engine so as to cope with the increase in the intake quantity of the air-fuel mixture at the high rotation range in the conventional internal combustion engine with a one-way valve of this kind, there occurred such occasion that the rigidity of the reed valve becomes high so that the opening degree of the reed valve becomes insufficient at the low rotation range.
In order to give a solution to such problem, for example, the peripheral portion of the reed valve was partially made thin so as to improve the efficiency to introduce the air-fuel mixture at the low rotation range. However, it was difficult to provide uniform efficiency to introduce the air-fuel mixture over the whole range from the low rotation range to the high rotation range so as to obtain such an engine capable of providing excellent average charging efficiency and high output.